A problem with boilers utilizing bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) is the efficient removal of impurities brought along with fuel, such as stones, metal material and other incombustible impurities, from the grate during the combustion process. At present, in connection with fluidized bed combustion the removal of coarse material from the grate is generally based on the natural flow of the material towards outlets. In addition to the shaping of the bottom of the grate it is also typical to use primary air to direct sand and said impurities to different channels, from where the sand and impurities are directed to an outlet or outlets. This type of technology is disclosed, inter alia, in publication WO 03/090919.
There are some problems in solutions according to prior art. For example, the directing with primary air is not always efficient enough. Especially in moving heavy materials the transfer effect of primary air is often too small. Heavy material may remain between nozzles and removing it from the grate may require regular shutdown of the boiler. Another problem with this solution is the heavy wearing of the primary air nozzles, because the bed material travelling over the air nozzles wears the nozzles. Yet another problem with this type of material removal is the unevenness of the removal. The flow profile of the bed material is difficult to make even, and removal especially from some edge areas of the grate may become problematic. In addition, efficient transfer of material requires a relatively large number of primary air nozzles, for example approximately 50 nozzles per m2, and therefore the implementation of this type of a solution may be expensive.